Ethylene-vinyl alcohol resins (hereinafter, may be abbreviated as EVOHs) exhibit superior gas barrier properties against oxygen, odors, flavors, etc., and are suitably used for e.g., packaging materials for foods and the like. With respect to such packaging materials, a heating treatment (retort treatment or boiling treatment) with hot water or water vapor is often carried out after filling the packaging materials with contents such as foods. However, the EVOH results in a problem through the heating treatment with hot water or water vapor for a long time period, involving resin whitening and/or deterioration of a capability of keeping the shape of the vacuum-packed contents.
To address such a drawback, molded articles (films, sheets, etc.) produced using, as a material, resin composition pellets produced by mixing the EVOH and a polyamide resin (hereinafter, may be abbreviated as “PA”) are used as a packaging material for a heating treatment. In these molded articles, the pellets are often used as a material of an intermediate layer of a multilayer structure, typically a multilayer structure including a layer composed of a polypropylene resin or the like.
However, the multilayer structure mentioned above also has a drawback that whitening (including blooming) may be caused partially or entirely on the surface of the multilayer structure due to the water absorption of the EVOH layer after the heating treatment.
To address the drawback, i.e., the whitening of the entire surface of the multilayer structure, Patent Document 1 (Japanese Unexamined Patent Application, Publication No. H4-131237) discloses a multilayer film including a layer composed of a composition containing an ethylene-vinyl alcohol copolymer resin, a polyamide resin, and a water-soluble or alcohol-soluble metal compound. The film can prevent the whitening of the entire surface of the film which may be caused upon adherence of a water droplet to the surface of the film for a long time period (which is a phenomenon generally referred to as rewhitening). However, the film cannot prevent generation of whitening (turbidity and/or whitening streaks in apart of the film) of a part of the film which may be caused after the heating treatment. In addition, melt-molding of the EVOH and the PA may have a drawback that a gel which is a thermally deteriorated matter of the EVOH and the PA is generated and consequently a pellet containing a residue of the gel is formed, leading to generation of seeds (dirt under paint) in a multilayer structure produced using the pellet as a material.
To address the aforementioned drawback, i.e., the generation of the seeds, Patent Document 2 (Japanese Unexamined Patent Application, Publication No. H7-97491) discloses a multilayer film that includes a layer composed of a composition containing an ethylene-vinyl alcohol copolymer resin, a polyamide resin, and at least two metal salts in which a metal in group II in periodic table is involved. However, a film produced using this resin composition has a drawback that the film is inferior in general appearance after a heating treatment, as also demonstrated in Comparative Example 8 described later.
Besides, to address the drawback, i.e., the generation of the seeds mentioned above, Patent Document 3 (Japanese Unexamined Patent Application, Publication No. 2009-242591) discloses a multilayer film having a layer produced using, as a material, a multilayer pellet constituted with a layer composed of, as a principal component, an ethylene-vinyl alcohol copolymer containing a metal salt, and a layer composed of a polyamide as a principal component. However, depending on the melt kneading condition of the pellet in the film production, the film may generate the turbidity and/or the whitening streaks in a part of the film after a heating treatment, similarly to the invention disclosed in Patent Document 1.